Method for transporting nitride ions in an electrochemical cell

ABSTRACT

A method for transporting nitride (N3−) ions in an electrochemical cell includes providing nitrogen to a first side of a solid electrolyte membrane to form nitride ions and transporting the nitride ions across the solid electrolyte membrane. The solid electrolyte membrane includes a metal nitride. The method may be used for ionically-mediated separation and/or compression of nitrogen or to form ammonia.

BACKGROUND

Ammonia (NH₃) has numerous uses including in fertilizers, as a CO₂-freehydrogen carrier, as a reactant to produce other nitrogen-containingcompounds, in cleaning products, scrubbing flue gas, etc. Due to itsnumerous uses, there is an immense demand for ammonia and interest innew technology related to its production.

Today, the Haber-Bosch process is the primary production method forproducing ammonia. In the Haber-Bosch process, nitrogen (N₂) is reactedwith hydrogen (H₂) under high temperature (e.g., 380-520° C.) andpressure (e.g., 120-220 bar) conditions in the presence of a metalcatalyst (e.g., iron oxide, FeO_(x)) to form ammonia:N₂+3H₂→2NH₃.

Unfortunately, the high temperature conditions result in thethermodynamic decomposition of ammonia, thereby reducing ammoniaconversion rate. Known ammonia production methods also typically requirelarge capital investment in equipment and maintenance costs due tomoving parts.

Process intensification in NH₃ production represents a domestic energysavings opportunity of 82 PJ/year. Domestic gas flaring in 2015 resultedin estimated energy losses of 300 PJ and emissions of 7 Mt_(CO2). Usingthis resource for large-scale inexpensive H₂ production is appealing.However, long-distance H₂ distribution costs make this impractical. IfH₂ produced from methane flares was converted to NH₃, which is mucheasier to transport as a liquid, then improved energy savings, emissionsavoidance, and energy security would result. Analysis of gas flaringdata suggests a methane (CH₄)→NH₃ plant size of 100 tons per day isrequired for this scheme, which is challenging to achieve usingHaber-Bosch process due to its poor cost scaling. If all US natural gasflares were converted to NH₃, this could supply 47% of the domesticproduction of ammonia (10.3 Mt_(NH3)) and make significant inroads intothe energy reduction opportunity (66 PJ/year), while simultaneouslyavoiding associated emissions of 10 Mt_(CO2)/year.

The unit processes involved in a typical ammonia synthesis productionfacility using the Haber-Bosch process are shown in FIG. 1. The processrelies on an inexpensive source of hydrogen, which is typically producedusing steam reforming. In a steam reformer SR, natural gas (methane,CH₄) is reacted with high-pressure steam (100 bar) over a transitionmetal catalyst (Ni) to produce H₂ according to the following reaction:CH₄+2H₂O→4H₂+CO₂.

The produced hydrogen is purified using pressure-swing adsorption toremove reactant gases and CO₂, and is compressed and passed into theHaber-Bosch synloop HB. Simultaneously, nitrogen is separated from airin an air separation unit ASU using either cryogenic distillation orpressure-swing adsorption and is compressed and enters HB.

The rate-limiting step in the ammonia synthesis pathway is the slowdissociation of the highly stable N₂ molecule. Although ruthenium (Ru)catalysts are known to be far more active in nitrogen reduction thanother catalysts such as platinum (Pt), they are susceptible to poisoningfrom H₂. Catalyst poisoning refers to the partial or completedeactivation of a catalyst by exposure to a chemical compound. In thiscase, catalyst poisoning prevents the use of Ru-based catalysts incommercial processes. The state of the art Haber-Bosch process has poorefficiency at small scale and requires large plants (greater than 1000tons per day) for economic feasibility. Small-scale Haber-Boschsynthesis would require drastic reductions of the synloop cost toeconomically harness the distributed nature of flared gas. Inparticular, integration of the ASU and HB synloop would lead to thecapital cost of two processes being integrated into a single processequipment, thereby reducing manufacturing cost.

Ammonia is also an important precursor in the synthesis of multiplechemicals such as hydrazine (N₂H₄), hydrogen cyanide (HCN), urea(N₂H₄CO), acetonitrile and phenol. Distributed ammonia synthesis couldbe a driver to reduce production costs in these processes.

The need for distributed generation of ammonia using small-scaleproduction facilities makes it desirable to develop new systems andmethods for producing ammonia with improved efficiency, especially atsmall scale. Electrochemical NH₃ synthesis approaches hold promise forhigh efficiency at small scale, but are currently hindered by lowNH₃-flux and resultant high cost. Work in this area to date involves theuse of proton (H⁺), carbonate ([CO₃]²⁻), hydroxide ([OH]⁻), oxide (O²⁻)or molten nitride (N³⁻) ion conductors. Protonic transport is typicallyassociated with a very poor faradaic efficiency (<2%) at highoverpotential due to parasitic H₂ evolution. Although lithium nitridedissolved in molten eutectics produces the highest NH₃ flux to date (20nmol cm⁻² s⁻¹) and faradaic efficiency (72%), it is still too low to becommercially feasible. Hybrid approaches have also been used to enhancethe rate of NH₃ production by combining electrochemical waterelectrolysis with a chemical N₂ reduction step, but are in a very earlystage of development, and offer much lower performance (7.6 mmol g⁻¹h⁻¹) than the established Haber-Bosch process (96.5 mmol g⁻¹ h⁻¹, ZA-5catalyst).

The reaction rate for ammonia synthesis is dependent upon theconcentration of the feed gases, and has an inverse dependence on the H₂and NH₃ partial pressure. The reaction rate is given by the followingequation below.r _(NH) ₃ =kP_(N) ₂ ^(0.8)P_(H) ₂ ^(−0.7)P_(NH) ₃ ^(−0.4)

Enhancement of the reaction rate requires operation under reducedpartial pressure or H₂ and high N₂ partial pressure. Molten electrolytesare particularly unfavored due to their inability to sustain adifferential pressure. For this reason. Operation at intermediatepressure is also desirable to reduce energy costs for compression ofreactant gases. solid electrolytes are typically used in solid-stateammonia synthesis. However, ceramic electrolytes are typically brittlematerials with low fracture toughness that cannot be machined or cuteasily, making them particularly susceptible to cracking due tomechanical stresses during assembly. Additionally, ionic oxide membranestypically require high operating temperatures (e.g., 800 to 1,000° C.)and are prone to thermal stresses during repeated periods of start-upand shutdown.

Tape-casting is the preferred route for fabricating ceramic membranes.This involves preparing ceramic slurry comprising ceramic particles inan appropriate liquid, casting it into a thin film (tape), which issintered at a high temperature. It is desirable to fabricate themembrane in-situ inside the electrochemical cell, such that this fragilematerial does not need further handling and is mechanically protected bythe cell. This enables the use of much thinner membranes, which enhancesenergetic performance and decreases system cost.

The tape cast membranes are typically many times larger than thecharacteristic particle size, which reduces their ionic area-specificresistance (ASR) and gas impermeability. For example, sinteredlithium-containing membranes typically have relatively low ionicconductivity relative to bulk or single-crystal films. Sinteredmembranes from ceramic materials also exhibit reduced electrochemicaland gas permeability compared to bulk films, all of which areundesirable characteristics for ion-transport membranes. It is desirableto fabricate a solid ceramic ion conductor as a fully dense membranethat enables high polycrystalline ionic conductivity. Additionally it isalso desirable to create thinner membranes with membrane thicknesscomparable to the characteristic length of a crystalline grain, suchthat conductivity approaches single-crystalline conductivity. Theimproved ionic conductivity and reduced thickness both result in animproved system performance and reduced capital costs.

Another process improvement in ammonia synthesis is the reduction in theoperating temperature. In this regard, the Haber-Bosch synloop suffersfrom an unfavorable thermodynamic equilibrium, and the single-passefficiency is typically low due to spontaneous ammonia decomposition.Intermediate temperature operation (<300° C.) is desirable to ensure ahigh NH₃ conversion efficiency.

Fast ion conducting membranes are also used for other applications thatare unrelated to ammonia synthesis. Specific examples involve stabilizedzirconia membranes (Y₂O₃, Sc₂O₃, Gd₂O₃ stabilizers) for oxide transportin fuel cells, lead fluoride (β-PbF₂) or lanthanum trifluoride (LaF₃)membrane for fluoride transport in ion-sensitive electrodes, silveriodide (AgI) for silver ion transport. There are currently no approachesfor the direct synthesis of bulk films of these materials. Densified,gas-impermeable membranes of these specific materials would enableimproved performance in the associated electrochemical processes.

BRIEF DESCRIPTION

The present disclosure relates to a solid electrolyte membrane andmethods for making and using the same.

Disclosed, in some embodiments, is a method for forming anelectrochemical stack comprising: assembling a precursor stackcomprising: a metal layer comprising a reactant metal; and introducing areactant gas into the precursor stack, wherein the reactant gas reactswith the reactant metal to form a solid electrolyte membrane in situ.

In some embodiments, the solid electrolyte membrane comprises a nitride;and wherein the reactant metal comprises at least one element selectedfrom the group consisting of lithium (Li), sodium (Na), magnesium (Mg),calcium (Ca), strontium (Sr), and barium (Ba).

The reactant metal may be lithium.

In some embodiments, the reactant gas comprises at least one gasselected from the group consisting of nitrogen (N₂) and ammonia (NH₃).

The solid electrolyte membrane may comprise a metal oxide or amixed-metal oxide.

In some embodiments, the reactant metal comprises one or more elementsselected from the group consisting of zirconium (Zr), yttrium (Y),scandium (Sc), cerium (Ce), and gadolinium; and the reactant gas isoxygen (O₂) or ozone (O₃).

The solid electrolyte membrane may comprise a fluoride, a sulfide, or aniodide; and the reactant gas may comprise fluorine (F₂), iodine (I₂), orhydrogen sulfide (H₂S).

In some embodiments, the reactant gas is introduced to theelectrochemical stack at a temperature in the range of from 25° C. to800° C. For example, the temperature may be in the range of from 100° C.to 325° C. for a lithium nitride membrane; from 25° C. to 800° C. for ayttria, scandia, ceria, gadolinia, or zirconia membrane; from 100 to325° C. for a lead fluoride or lanthanum fluoride membrane; or 185 to325° C. for a silver iodide membrane.

The reactant gas may be introduced to the electrochemical stack at apressure in the range of from 1 bar to 10 bar.

In some embodiments, the precursor stack further comprises a firstcatalyst electrode layer and a second catalyst electrode layer; whereinat least one of the first catalyst electrode layer and the secondcatalyst electrode layer comprises a ruthenium (Ru) catalyst.

Disclosed, in other embodiments, is a method for forming a solidelectrolyte membrane, the method comprising: providing a metal layercomprising a reactant metal; and providing a reactant gas to react withthe reactant metal to form the solid electrolyte membrane.

In some embodiments, the solid electrolyte membrane is formed in situ inan electrochemical cell.

The reactant metal may be selected from the group consisting of lithium(Li), magnesium (Mg), aluminum (Al), calcium (Ca), strontium (Sr),barium (Ba), or a mixture thereof.

In some embodiments, the reactant metal is lithium.

The reactant gas may comprise at least one gas selected from the groupconsisting of nitrogen (N₂) and ammonia (NH₃).

A solid electrolyte membrane produced by the method is also disclosed.

Disclosed, in further embodiments, is a system comprising a solidelectrolyte membrane for an electrochemical stack; wherein the solidelectrolyte membrane is formed by an in situ chemical reaction between areactant metal and a reactant gas when assembled within theelectrochemical stack.

The solid electrolyte formed may be lithium nitride.

Disclosed, in additional embodiments, is an electrochemical device thatforms the solid electrolyte membrane in situ.

Also disclosed is a method for transporting nitride ions (N³⁻) across asolid electrolyte (e.g., a solid electrolyte membrane) comprising ametal nitride. The method includes providing nitrogen to a first side ofthe solid electrolyte, wherein the nitrogen reacts to form nitride ions;and transporting nitride ions across the solid electrolyte. The nitrogenmay be provided in any suitable form, including but not limited to N₂and NH₃.

In some embodiments, the metal nitride comprises at least one metalselected from the group consisting of lithium (Li), magnesium (Mg),aluminum (Al), calcium (Ca), strontium (Sr), barium (Ba), sodium (Na),or a mixture thereof.

The solid electrolyte may comprise lithium nitride.

In some embodiments, the solid electrolyte is located between a firstcatalyst electrode layer and a second catalyst electrode layer.

In some embodiments, at least one of the first catalyst electrode layerand the second catalyst electrode layer comprises a ruthenium (Ru)catalyst. The ruthenium catalyst may be an alkali promoted rutheniumcatalyst.

In some embodiments, the metal nitride electrolyte was formed in situ inthe electrochemical cell.

Disclosed, in other embodiments, is a method for producing at least onenitrogen-containing compound in an electrochemical cell; wherein theelectrochemical cell comprises a solid electrolyte membrane, wherein thesolid electrolyte membrane comprises a metal nitride and can transportnitride ions; the method comprising: providing nitrogen to a first sideof the membrane, wherein the nitrogen reacts to form nitride ions (N³⁻),wherein the solid metal nitride electrolyte is capable of transporting(e.g., permeable to) nitride ions (N³⁻); and providing a reactant to asecond side of the membrane, wherein the reactant reacts with nitrideions (N³⁻) to form the at least one nitrogen-containing compound.

The cell may be located in an electrochemical stack comprising aplurality of cells.

In some embodiments, the electrochemical cell is supplied with ahydrogen source and a nitrogen source; the electrochemical cell isintegrated with an electricity source; and the method electrochemicallyproduces ammonia.

In some embodiments, the metal nitride is lithium nitride.

The metal nitride may be lithium nitride, the electrochemical cell mayfurther include a first catalyst electrode layer and a second catalystelectrode layer, and at least one of the first catalyst electrode layerand the second catalyst electrode layer may comprise a ruthenium (Ru)catalyst. Optionally, the solid electrolyte membrane may be impermeableto gases. Optionally, only one of the first catalyst electrode layer andthe second catalyst electrode layer comprises the ruthenium (Ru)catalyst.

In some embodiments, the electrochemical cell is operated at atemperature in the range of from 150° C. to 250° C.

The nitrogen may be provided to the first catalyst electrode layer at apressure in the range of from 1 bar to 10 bar.

In some embodiments, the method electrochemically produces hydrazine,hydrazoic acid, or hydrogen cyanide.

Disclosed, in further embodiments, is a method for separating orcompressing nitrogen (N₂) in an electrochemical cell; wherein theelectrochemical cell comprises in sequence: a first current collector;optionally a first gas diffusion layer; optionally a first catalystelectrode layer; a solid electrolyte membrane, wherein the solidelectrolyte membrane comprises a metal nitride and can transport nitrideions; optionally a second catalyst electrode layer; optionally a secondgas diffusion layer; and a second current collector; the methodcomprising: providing a composition comprising nitrogen (N₂) at a firstpressure or partial pressure to a first side of the membrane, whereinthe nitrogen reacts to form nitride ions (N³⁻), and wherein the nitrideions react at a second side of the membrane to form nitrogen (N₂); andremoving nitrogen (N₂) at a second pressure or partial pressure from theelectrochemical stack through an outlet in the second current collector;wherein the second pressure or partial pressure is greater than thefirst pressure or partial pressure.

The nitrogen may be provided to the first side of the membrane in agaseous mixture that further comprises at least one additional gas.

In some embodiments, the first catalyst electrode layer comprisesruthenium; and wherein the second catalyst electrode layer comprisesruthenium.

The composition may consist essentially of nitrogen; and the method maycompress nitrogen.

Disclosed, in additional embodiments, is an electrochemical system fortransporting nitride ions (N³⁻). The system includes a solid electrolytemembrane comprising a metal nitride and capable of transporting nitrideions.

These and other non-limiting characteristics are more particularlydescribed below.

BRIEF DESCRIPTION OF THE DRAWINGS

The following is a brief description of the drawings, which arepresented for the purposes of illustrating the exemplary embodimentsdisclosed herein and not for the purposes of limiting the same.

FIG. 1 is a flow chart showing the process steps involved in ammoniasynthesis using the Haber-Bosch process.

FIG. 2 is a flow chart showing a method for producing an electrochemicalstack in accordance with some embodiments of the present disclosure.

FIG. 3 is a cross-sectional view of an electrochemical stack inaccordance with some embodiments of the present disclosure.

FIG. 4 is an exploded view of another electrochemical stack inaccordance with some embodiments of the present disclosure.

FIG. 5 schematically illustrates ammonia synthesis using a solid-statenitride conducting membrane in accordance with some embodiments of thepresent disclosure.

FIG. 6 illustrates system integration of a modular electrochemicalammonia generator stack (MEGA) with a H₂ production process (steamreforming, SR), an air separation process (cryogenic distillation) andan electricity generation process (gas turbine) in accordance with someembodiments of the present disclosure.

FIG. 7 schematically illustrates nitrogen compression using anitride-conducting solid membrane in accordance with some embodiments ofthe present disclosure.

FIGS. 8A and 8B show a comparison of preliminary electrochemicalimpedance spectroscopy measurements of Li₃N ionic conductivity withexisting literature and the crystal structure of α-Li₃N, respectively.

FIG. 9 includes photographs showing the synthesis of a 1.5 mm thicklithium nitride membrane by reacting Li in 1 bar N₂ at 220° C. in whicha prototype electrochemical stack (3 cm² active area) was fabricatedusing a cathode catalyst (4 mg/cm² Pt/Ru), an anode catalyst (4 mg/cm₂Pt) and Li metal. The Li metal was reacted under passage of N₂ to formthe lithium nitride membrane in situ.

FIG. 10 illustrates cyclic voltammogram (10 mV/s) demonstrating NH₃synthesis at 180° C. in a preliminary cell operated with Pt—Ru catalyst(N₂) and Pt catalyst (H₂). The NH₃ produced was detected using a 100 ppmsensor, which saturated at higher values of applied potential.

FIG. 11 is a graph showing some of the results of the proof-of-conceptdemonstration for nitrogen generation.

FIG. 12 is a photograph of a portion of a disassembled, layeredelectrochemical cell formed in accordance with one of the examples ofthe present disclosure.

FIG. 13 is a graph showing the change in pressure over time when anelectric field was applied to the device of FIG. 12.

FIG. 14 is a graph showing the change in pressure over time when theelectric field applied in FIG. 13 was reversed.

FIG. 15 illustrates an exemplary thermal profile in accordance with someembodiments of the present disclosure.

FIG. 16 illustrates an exemplary chemical (gas) profile in accordancewith some embodiments of the present disclosure.

FIG. 17 in a scanning electron microscope (SEM) image of a membraneproduced according to one of the examples at 37× magnification and 1000×magnification (inset).

FIG. 18 is an EDX linescan of nitrogen content across a cross-section ofa membrane made according to one of the examples.

FIG. 19 is a graph illustrating Faradaic efficiency and ammonia flux inan electrochemical cell made in accordance with one of the examples.

DETAILED DESCRIPTION

A more complete understanding of the membranes, stacks, systems, andmethods disclosed herein can be obtained by reference to theaccompanying drawings. These figures are merely schematicrepresentations based on convenience and the ease of demonstrating theexisting art and/or the present development, and are, therefore, notintended to indicate relative size and dimensions of the assemblies orcomponents thereof.

Unless otherwise defined, all technical and scientific terms used hereinhave the same meaning as commonly understood by one of ordinary skill inthe art. In case of conflict, the present document, includingdefinitions, will control. Preferred methods and materials are describedbelow, although methods and materials similar or equivalent can be usedin practice or testing of the present disclosure. The materials,methods, and articles disclosed herein are illustrative only and notintended to be limiting. For example, lithium nitride solid electrolytesare discussed throughout this application, particularly in the Examplessection. However, reactant metals other than lithium are alsocontemplated. Similarly, ammonia production is discussed throughout thisapplication but other applications are also contemplated.

The singular forms “a,” “an,” and “the” include plural referents unlessthe context clearly dictates otherwise.

As used in the specification and in the claims, the term “comprising”may include the embodiments “consisting of” and “consisting essentiallyof.” The terms “comprise(s),” “include(s),” “having,” “has,” “can,”“contain(s),” and variants thereof, as used herein, are intended to beopen-ended transitional phrases that require the presence of the namedingredients/steps and permit the presence of other ingredients/steps.However, such description should be construed as also describingcompositions, mixtures, or processes as “consisting of” and “consistingessentially of” the enumerated ingredients/steps, which allows thepresence of only the named ingredients/steps, along with any impuritiesthat might result therefrom, and excludes other ingredients/steps.

Unless indicated to the contrary, the numerical values in thespecification should be understood to include numerical values which arethe same when reduced to the same number of significant figures andnumerical values which differ from the stated value by less than theexperimental error of the conventional measurement technique of the typeused to determine the particular value.

All ranges disclosed herein are inclusive of the recited endpoint andindependently combinable (for example, the range of “from 2 to 10” isinclusive of the endpoints, 2 and 10, and all the intermediate values).The endpoints of the ranges and any values disclosed herein are notlimited to the precise range or value; they are sufficiently impreciseto include values approximating these ranges and/or values.

As used herein, approximating language may be applied to modify anyquantitative representation that may vary without resulting in a changein the basic function to which it is related. Accordingly, a valuemodified by a term or terms, such as “about” and “substantially,” maynot be limited to the precise value specified, in some cases. Themodifier “about” should also be considered as disclosing the rangedefined by the absolute values of the two endpoints. For example, theexpression “from about 2 to about 4” also discloses the range “from 2 to4.” The term “about” may refer to plus or minus 10% of the indicatednumber. For example, “about 10%” may indicate a range of 9% to 11%, and“about 1” may mean from 0.9-1.1.

For the recitation of numeric ranges herein, each intervening numberthere between with the same degree of precision is explicitlycontemplated. For example, for the range of 6-9, the numbers 7 and 8 arecontemplated in addition to 6 and 9, and for the range 6.0-7.0, thenumber 6.0, 6.1, 6.2, 6.3, 6.4, 6.5, 6.6, 6.7, 6.8, 6.9, and 7.0 areexplicitly contemplated.

The present disclosure relates to electrochemical stacks including asolid electrolyte membrane. Methods for making and using theelectrochemical stacks and solid electrolyte membranes are alsodisclosed. In some embodiments, the membrane comprises a metal nitride,a metal oxide or a mixed-metal oxide, a metal fluoride, a metal iodide,or a metal sulfide.

The lithium nitride (e.g., Li₃N) membranes of the present disclosure actas a solid nitride-ion (N³⁻) conducting electrolyte to facilitate ionelectromigration under an applied electric field. The membrane is a goodionic conductor at intermediate temperatures (σ_(453K)=4.5 mS cm⁻¹),with a high nitride ion mobility (t_(N3−)˜0.4).

Metals are generally easier to machine than metal oxides due to theirgreater sectility and ductility. It is easier to assemble anelectrochemical stack starting with a reactive base metal, and minimizethe detrimental effects of mechanical stresses during assembly. Theproduction of the ion transport membrane through an in situ reactionreduces the likelihood of membrane failure as a result of physicalmembrane cracking.

Additionally, membranes made using the systems and methods of thepresent disclosure retain the properties of bulk films, and are morepolycrystalline in nature. This results in lithium nitride membraneswith ionic conductivities approaching single-crystal values, which aresignificantly higher than values for sintered lithium.

The membrane can be combined with catalyst electrodes to form a membraneelectrode assembly (MEA), which is the building block of the highlymodular electrochemical stack. The membrane is synthesized by reactingLi metal sheet and N₂ gas, which occurs in situ once the membraneelectrode assembly (MEA) has been assembled, which will lend itself toreel-to-reel processing, simplifying stack assembly and dramaticallyreducing fabrication costs.

The electrochemical stacks of the present disclosure may be used toproduce a variety of nitrogenous chemicals when the membrane comprises ametal nitride. For example the electrochemical stack may producehydrazine (N₂H₄), hydrazoic acid (HN₃), or ammonia, which may beproduced through the overall reactions below. The reaction products aredetermined by the choice of catalyst and operating conditions ofvoltage, gas composition and temperature. This enables the on-demandsynthesis of these chemicals using much less hazardous intermediates.3N₂+H₂→2NH₃2N₂+H₂→N₂H₄N₂+3H₂→2NH₃

The intermediate temperature operation and the use of a N³⁻ conductingsolid electrolyte avoids the two primary problems in current ammoniageneration technologies:

1. low ammonia conversion due to thermodynamic ammonia decomposition athigh temperatures in the Haber-Bosch Process; and

2. low ammonia conversion due to parasitic hydrogen evolution inelectrochemical processes.

The modular stack design has no moving parts and a much lower capitalcost than state-of the-art ammonia generation processes such asHaber-Bosch for small-scale NH₃ generation. This small-scale ammoniageneration enables the harnessing of energy that is currently lostduring natural gas flaring or curtailed renewables.

During operation, feed gases (N₂/H₂) enter the Modular ElectrochemicalGeneration of Ammonia (MEGA) stack, maintained at intermediatetemperature (e.g., from about 200 to about 250° C.). The thermodynamicdecomposition of NH₃ at this temperature is avoided by operating at amoderate hydrostatic pressure (e.g., less than about 5 bar). Theseparation of the feed gases using a solid membrane enables the use ofN₂-selective Ru catalysts without risk of H₂ poisoning. At the cathode,N₂ is reduced (½N₂+3e⁻→N³⁻) and N³⁻ migrates across the membrane andreacts with H₂ (3/2H₂+N³⁻→NH₃+3e⁻) to produce NH₃.

MEGA's modularity is well-suited for use in conjunction with wind farmsor gas flares, which vary considerably by location and intensity. TheMEGA stack may be integrated with a small-scale steam reformer and airseparation unit.

A preliminary cost model to assess the production cost assumes a scalingexponent of 0.6 for small-scale steam reformer (SR) and air separationunit (ASU) plants and no cost for flared gas. The resulting NH₃ costusing MEGA compares favorably to a much larger Haber-Bosch facility andallows reduction of NH₃ transportation costs for manufacture offertilizer. For example, the flared gas in North Dakota could be used tosupply >40% of the NH₃ demand in the Corn Belt. Process optimizationduring the integration of MEGA, SR, and ASU could result in even morefavorable energetics and economics.

FIG. 2 is a flow chart illustrating an exemplary method for producing asolid electrolyte membrane in situ in an electrochemical stack.Initially, a metal metal layer 105 is provided. In the depictedembodiment, the metal layer is provided in the form of a roll. The metallayer 105 is cut and laminated 160 with a first catalyst layer 130, asecond catalyst layer 135, and sealing gaskets 132, 137 to form amembrane electrode assembly. The membrane electrode assembly isassembled 165 into a reactive metal stack by the provision of currentcollectors 150, 155 (e.g., bipolar plates). Finally, reactant gas (e.g.,nitrogen gas, ammonia gas, or a nitrogen oxide gas) at relatively lowtemperature and pressure (compared to the Haber-Bosch process) isprovided to the reactive metal stack and reacts 170 with the reactivemetal layer to form a solid electrolyte membrane 120 in situ in theelectrochemical stack.

In some embodiments, the solid electrolyte membrane comprises a nitride,an oxide, an iodide, or a fluoride ion conductor.

The solid electrolyte membrane is formed by reacting a reactant metalwith a reactant gas. The reactant metal can include one or more of thefollowing elements: lithium (Li), sodium (Na), magnesium (Mg), calcium(Ca), strontium (Sr), zirconium (Zr), yttrium (Y), scandium (Sc), cerium(Ce), and gadolinium (Gd).

In some embodiments, the reactant gas comprises nitrogen (N₂), ammonia(NH₃), oxygen (O₂), ozone (O₃), hydrogen (H₂), or fluorine (F₂).

When the membrane comprises a nitride, the reactant metal may containlithium (Li), sodium (Na), magnesium (Mg), calcium (Ca), and/orstrontium (Sr) and the reactant gas may be nitrogen (N₂), ammonia (NH₃),or hydrazine (N₂H₄).

When the membrane comprises an oxide, the reactant metal may containzirconium (Zr), yttrium (Y), scandium (Sc), cerium (Ce), and/orgadolinium (Gd) and the reactant gas may be oxygen (O₂) or ozone (O₃).

When the membrane comprises a fluoride, the reactant gas may be fluorine(F₂) and the reactant metal may be lanthanum (La) or lead (Pb).

When the membrane comprises an iodide, the reactant gas may be iodine(I₂), and the reactant metal may be silver (Ag).

When the membrane comprises a sulfide, the reactant gas may be hydrogensulfide (H₂S) and the reactant metal may be silver (Ag).

FIG. 3 illustrates a non-limiting embodiment of an electrochemical stack100 in accordance with some embodiments of the present disclosure. Thestack 100 includes a solid electrolyte membrane 120 (e.g., a solid metalnitride electrolyte membrane such as a lithium nitride membrane).Catalyst electrode layers 130, 135 are provided on each side of theelectrolyte membrane 120. The first catalyst electrode layer 130 islocated between the electrolyte membrane 120 and a first, microporouslayer 140. The second catalyst electrode layer 135 is located betweenthe electrolyte 120 and a second, microporous layer 145. The firstmicroporous layer 140 is located between the first catalyst electrodelayer 130 and a first current collector 150. The second microporouslayer 145 is located between the second catalyst electrode layer 135 anda second current collector 155. The current collectors 150, 155 includeinlets and outlets 152, 157 for introducing gases.

The electrolyte 120 may have a thickness in the range of from about 50μm to about 2,000 μm, including from about 50 μm to about 250 μm andfrom about 50 μm to about 100 μm.

The reactant metal of the metal nitride electrolyte 120 may include atleast one metal selected from lithium, magnesium, aluminum, scandium,gadolinium, calcium, cerium, and barium.

In some embodiments, the nitride is a mixed metal nitride (i.e.,contains more than one metal). The mixed nitride may contain lithium incombination with one or more metals selected from sodium, potassium,magnesium, aluminum, scandium, gadolinium, calcium, cerium, and barium.

In particular embodiments, the nitride may be a lithium-aluminum nitride(Li₃AlN₂).

The performance of the electrochemical cell relies on the electrolytemembrane to have a high ionic conductivity for the nitride ion (e.g.,greater than 1 mS/cm), low area-specific resistance (e.g., less than 100ohm-cm²), and poor electrical conductivity. Additionally, the membranehas very low permeability to gases in order to prevent mixing of thefeed gases.

The catalyst layers 130, 135 contain a conductive metal catalyst.

The catalyst layers may have the same composition or differentcompositions. For example, in applications wherein only one side of themembrane is exposed to hydrogen, the catalyst layer of the unexposedside may contain ruthenium whereas the catalyst layer of the exposedside does not contain ruthenium.

In some embodiments, the catalyst layers 130, 135 independently containat least one metal selected from gold, iridium, palladium, platinum, andruthenium.

One or both catalyst layers 130, 135 may consist of ruthenium. In otherembodiments, one or both catalyst layers contains ruthenium incombination with at least one metal selected from gold, iridium,palladium, and platinum.

In some embodiments, the catalyst is an alkali promoted catalyst.

The thicknesses of the catalyst layers 130, 135 may be the same ordifferent. The catalyst layer may comprise a catalyst coated gasdiffusion electrode (e.g., 250 μm carbon paper or 400 μm carbon cloth).The thickness depends upon the choice of electrode used.

The catalyst layer may ensure a high specific surface area (e.g., fromabout 10 to about 60 m²/g).

The catalyst-coated gas diffusion electrode enhances the reaction rateby increasing the three-phase (gas, catalyst, and electrolyte) contactarea available for reaction while simultaneously providing a path forthe easy collection of electrons produced during the reaction. For thisreason, the gas diffusion electrode is made of a good electricalconductor such as carbon, covered with a layer of carbon felt to ensurea high surface area. The electrode surface may occasionally bepost-treated with a binder (e.g., PTFE) to ensure mechanical stability.

The microporous layers 140, 145 (also known as gas transport or gasdiffusion layers) may be the same or different in terms of compositionand thickness. These layers are porous to enable transport of gases.

In some embodiments, each microporous layer independently contains aporous material selected from carbon cloth, carbon felt, carbon paper,metal mesh, and metal foam (e.g., nickel).

The gas transport layer has adequate porosity to ensure high gaspermeability (e.g., less than 10 s), low electrical area-specificresistance (less than 0.013 ohm-cm², and high tensile strength (greaterthan 5 N/cm).

In some embodiments, the gas transport layer is covered with amicroporous layer (e.g., less than 50 μm thick) made from carbon black.The purpose of this layer is to ensure good areal contact between thegas transport layer and, in PEM fuel cell, encourage the wick-off ofproduced water. In this embodiment, the microporous layer reduces theelectrode's porosity in the vicinity of the solid electrolyte, creatinga high capillary pressure to ensure that the lithium nitride does notcompletely submerge the catalyst.

In some embodiments, the catalyst electrode layers and/or gas transportlayers are optional.

In some embodiments, the current collectors are in physical contact withthe electrolyte membrane.

In general, the current collectors 150, 155 exhibit good electricalconductivity, high thermal conductivity, high chemical resistance, highcorrosion resistance, mechanical stability against compressive forces,and low permeability to the gases used in a particular application(e.g., hydrogen).

The current collectors may contain one or more conductive materialsselected from graphite, metals, and metal alloys. In some embodiments,the metal is stainless steel (e.g., Type 304 and/or Type 316), copper,or titanium.

The current collectors generally include one or more flow fields forenhanced gas distribution. In some embodiments, the flow field(s) is/areserpentine.

The current collectors generally have a high electrical conductivity(e.g., greater than 10⁴ S/m), low gas permeability and chemicalstability towards the feed and product gases, high tensile strength toallow operation at intermediate pressure (e.g., greater than 5 bar), andgood machinability to enable the machining of flow fields.

In some embodiments, each current collector has a thickness within therange of from about 1 to about 5 mm.

The electrochemical stack 100 of FIG. 3 includes a single cell. However,it should be understood that the electrochemical stacks of the presentdisclosure may also include a plurality of cells. FIG. 4 is an explodedview illustrating an electrochemical stack 200 with two cells. Eachmembrane electrode assembly 210 includes a solid electrolyte membraneand catalyst electrodes and is sandwiched between two current collectors250. In some embodiments, a stack module contains from about 50 to about100 cells. For example, a 50-cell stack (20 cm×20 cm) operating at acurrent of 250 mA/cm² may be able to produce 1 kg of ammonia per hour.

In some embodiments, the electrochemical stack is used to produceammonia. A non-limiting example of an exemplary ammonia productionmethod is schematically shown in FIG. 5. The system includes a solid,metal nitride electrolyte 320, first and second catalyst electrodelayers 330, 335, and first and second current collectors 350, 355. Inthe depicted embodiment, the electrolyte 320 is a lithium nitrideelectrolyte, the first catalyst electrode layer 330 is a rutheniumcatalyst layer, and the second catalyst electrode layer 335 is aplatinum catalyst layer. Nitrogen gas is provided through the firstcurrent collector 350 via inlet 374 and hydrogen gas is provided throughthe second current collector 355 via inlet 372. Unreacted nitrogen maybe removed via outlet 376 and is optionally recycled. Nitride ions (N³⁻)are generated at the first catalyst layer 330. The electrolyte 320 isselectively permeable to the nitride ions, which pass therethrough. Thenitride ions reaching the second catalyst layer 335 react with hydrogento form ammonia. Ammonia generated in the electrochemical stack may beremoved from the system via outlet 378. The solid electrolyte 320 is notpermeable to hydrogen. Therefore, the hydrogen provided via inlet 372does not reach the ruthenium catalyst 330. Thus, the solid electrolyte320 prevents the deleterious poisoning of the catalyst 330.

The systems and methods of the present disclosure enable distributedgeneration of ammonia from an underutilized energy source (e.g., naturalgas flares, curtailed or stranded renewables such as wind farms andsolar farms). During operation, the stack is maintained at anintermediate temperature and the reactant gases (N₂ and H₂) enter thestack at moderate pressure. In some embodiments, the intermediatetemperature is less than about 250° C. The temperature may be in therange of from about 150° C. to about 250° C. In some embodiments, themoderate pressure is less than about 10 bar. The pressure may be in therange of from about 1 bar to about 10 bar. The nitrogen is typicallyseparated from air by cryogenic distillation, membrane separation orpressure swing adsorption. The hydrogen is typically obtained by steamreforming of natural gas at flared gas site or through waterelectrolysis at a renewable energy farm. When an active voltage isapplied, N₂ is reduced at the cathode (i.e., first catalyst layer 330 inFIG. 5) at the catalyst surface to form nitride ions (N³⁻):N₂+6e ⁻→2N³⁻

The resulting N³⁻ migrates to the anode (i.e., second catalyst layer 335in FIG. 5) where it reacts with H₂ to generate ammonia:3H₂+2N³⁻→2NH₃+6e ⁻.

FIG. 6 is a flow chart illustrating an exemplary system which includes amodular electrochemical ammonia generator stack MEGA. Steam reformer SRmay be similar to or different from the steam reformer of FIG. 1 andprovides hydrogen to the MEGA stage. Air separation unit ASU may besimilar to or different from the air separation unit of FIG. 1 andprovides nitrogen to the MEGA stage. A TURBINE stage provides energy tothe MEGA stage. In addition or as an alternative to the turbine, energymay be provided by wind or solar power. The MEGA stage produces ammoniaat a considerably lower temperature and pressure in comparison to the HBstage in FIG. 1.

In other embodiments, the electrochemical stack is used to separatenitrogen from an inert gas and/or compress nitrogen. A non-limitingexample of an exemplary nitrogen compressing method is schematicallyshown in FIG. 7. The system includes a solid, metal nitride electrolyte420, first and second catalyst electrode layers 430, 435, and first andsecond current collectors 450, 455. In the depicted embodiment, theelectrolyte 420 is a lithium nitride electrolyte and the first andsecond catalyst electrode layers 430, 435 are ruthenium catalyst layers.In this case, ruthenium can be used in both catalyst layers 430, 435because catalyst poisoning from hydrogen is not an issue since bothfeeds 472, 474 provide nitrogen to the electrochemical stack. The firstinlet 472 provides low pressure nitrogen and the second inlet 474provides high pressure nitrogen. Low pressure nitrogen is also recycledvia recycle line 476. High pressure nitrogen leaves the system viaoutlet 478. Nitrogen reacts at the first catalyst layer 430 (i.e.,cathode) to form nitride ions (N³⁻) which the electrolyte 420 ispermeable to. The nitride ions react in a reversible reaction at thesecond catalyst layer 435 (i.e., anode) to produce more nitrogen,thereby increasing the pressure of nitrogen in the second side of thecell. The reversible reaction at the second catalyst layer is shownbelow:N₂+6e ⁻

N³⁻.

As a result, the gas pressure downstream from the N₂ inlet increases.When multiple stages are stacked, the pressure of the product N₂ gas canbe increased substantially (e.g., to greater than about 100 bar at 250°C.). These conditions are adequate for nitrogen to exist as asupercritical fluid, and the resulting product can be a N₂ feed forapplications such as supercritical drying, or purification of porousmaterials.

The systems and methods of the present disclosure may be used toseparate N₂|inert gas mixtures. Solid electrolyte membranes afford amodular structure that has better cost-scaling for small-scale gasprocessing applications. One example is the purification of noble gases(Ar, Kr, Ne, Xe). This is currently achieved using adsorption columns orcryogenic distillation, both of which require high installation andoperation costs. Polymeric membranes for separation of N₂|He suffer fromlow He selectivity or low flux. The Knudsen selectivity for these gasesis low, and polymeric membranes to separate N2|He mixtures requiremultiple stages and a substantial membrane area. Ion-selective membranesoffers much higher selectivity, and are desirable to improve the flux inseparation of inert gases.

The compressed N₂ may also be used in the production of ammonia.

The separation/compression systems and methods of the present disclosureovercome the deficiencies of the prior art for small scale applications.They may be useful for on-site chemical production, for remote chemicalproduction based on the availability of inexpensive reagents at smallscale, for enabling continued production during large plant maintenance,for modular expansion of existing plants, or for low-capital expenditureplants that can scale production based on real-time costs of reagents.

The portable electrochemical nitrogen separators/compressors may be usedfor delivering portable compressed nitrogen for filling tires inautomobiles. Oxide ion transport membranes need a high operatingtemperature to achieve this task.

The portable electrochemical nitrogen separators/compressors may also beused for delivering high purity N₂ to maintain an inert atmosphere(e.g., for storage of combustible or reactant products such aspetroleum).

The systems can also function as portable add-ons to ovens and gloveboxes to maintain inert conditions without requiring industrial air.

In other embodiments, the electrochemical stacks of the presentdisclosure may be used to detect ammonia.

FIG. 8A is a graph showing a comparison of preliminary electrochemicalimpedance spectroscopy measurements of Li₃N ionic conductivity withexisting literature. FIG. 8B illustrates the crystal structure ofα-Li₃N.

One feature of the systems and methods of the present disclosure is therapid nitride ion (N³⁻) transport in a solid electrolyte at moderatetemperatures, which enables efficient NH₃ generation while avoidingparasitic H₂ evolution or thermal decomposition of NH₃. This is due tothe unique crystal structure of α-Li₃N (FIG. 8B), which constitutessheets of (Li₂N)⁻ and mobile Li⁺ channels. As a result, the material waspreviously thought of as a single-ion conductor. Lithium nitride hasbeen known to be an effective Li⁺ conductor, and solid-state N³⁻transport has been demonstrated in proof-of-concept experimentsdiscussed in the Examples section of this application. The disruptivemanufacturing approach may involve roll-to-roll assembly of Li metal andthe other layers into a complete membrane electrode assembly (MEA),followed by in situ membrane formation by Li nitridation:3Li+½N₂→Li₃N.

This will dramatically reduce fabrication costs by avoiding moreexpensive membrane fabrication methods such as tape-casting and vapordeposition, enabling the fabrication of a solid ceramic ion conductor asa fully dense membrane with ionic conductivity and reduced gaspermeability.

This also enables the fabrication of membranes at thickness andprocessing conditions that allow for the characteristic length of acrystalline grain to be similar to or greater than the thickness of thefilm, such that conductivity approaches single-crystalline conductivity.

The ability to fabricate the membrane inside the electrochemical cell,such that this fragile material does not need further handling and ismechanically protected by the cell, enables the use of much thinnermembranes than would otherwise be possible, which in turn decreases theresistance of the cell, increases the current density and efficiency,and reduces the both the capital cost and electricity cost of thechemical that the stack produces.

The systems and methods of the present application have numerousadvantages over the prior art. One advantage is that the modularelectrochemical stack has no moving parts (low maintenance costs) andscales very well with size (high efficiency at any size). This makes itparticularly well suited for small scale ammonia generation (e.g., atflared gas sites and wind farms).

Another advantage is that nitride ion transport prevents parasitic H₂evolution along with N₂ reduction.

An additional advantage is that current density may be comparable toHaber-Bosch by membrane optimization (using thinner membrane, reducedpolycrystallinity) and improvements in N₂-reduction catalysts.

A further advantage is that the metal nitride provides a solid barrierwhich prevents H₂ crossover and poisoning of the N₂-selective Rucatalyst.

Still another advantage is the process operates at intermediatetemperatures and can potentially avoid thermodynamic decomposition ofammonia.

Yet another advantage is that the simple manufacturing method avoids theneed for expensive tape casting or chemical vapor deposition fabricationprocesses.

The following examples are provided to illustrate the devices andmethods of the present disclosure. The examples are merely illustrativeand are not intended to limit the disclosure to the materials,conditions, or process parameters set forth therein.

Examples

Formation of Lithium Nitride Layer

An experiment was conducted to demonstrate the feasibility of membranefabrication by reacting lithium (Li) metal in a nitrogen (N₂) atmosphere(1 bar) at 220° C. At this temperature, the lithium rapidly reacted withnitrogen (Li+3/2 N₂→Li₃N) to yield a dark colored product. The resultsare shown in FIG. 9 which shows a lithium metal layer prior to exposureto the nitrogen environment (0 minutes; left photograph); after 10minutes of exposure, before the lithium nitride layer was fully formed(middle photograph); and after the formation of the lithium nitridelayer (90 minutes; right photograph). The lithium nitride membrane had athickness of about 1.5 mm.

Fabrication of Prototype Electrochemical Stack

A prototype electrochemical stack was fabricated using a cathodecatalyst (4 mg/cm² Pt/Ru), an anode catalyst (4 mg/cm² Pt), and lithiummetal. The lithium metal was reacted under the passage of nitrogen gasto form the lithium nitride membrane in situ. Forming the membrane insitu is advantageous because the lithium metal layer is more flexibleand simplifies assembly, whereas the lithium nitride layer is lessflexible, more difficult to bend, and more likely to be damaged ifassembly is performed after the membrane has formed.

Ammonia Generation and Nitrogen Evolution Using Electrochemical Stack

Experiments were performed to assess the feasibility of the proposedelectrochemical stack for ammonia generation. The temperature dependenceof the ionic conductivity is compared to existing literature data inFIG. 8A. It is known that single-crystal Li₃N has a highly anisotropicconductivity. The preliminary membrane has an overall conductivity thathas an intermediate value between that of the parallel (cross-plane) andperpendicular (in plane) ionic conductivities. It is also noted that theelectronic conductivity of the material is at least 10⁴× higher,indicating this to be a pure ionic conductor.

The proof of concept experiments to demonstrate ammonia generation andnitrogen evolution were performed using a 3 cm² electrochemical stack.Ammonia was generated using a feed of H₂ (5.07% in Ar, 80 mL min⁻¹) andN₂ (100 mL min⁻¹) at 180° C. The ammonia produced was detected using anelectrochemical ammonia sensor (100 ppm detection limit). Theexperimental results conclusively demonstrate the synthesis of ammoniaat rates sufficient enough to saturate the ammonia sensor. The maximumcurrent obtained was 8.64 mA/cm² at 1.5 V, which corresponds to anammonia synthesis flux of 30 nmol cm⁻² s⁻¹ (90% faradaic efficiency).The results of the nitrogen generation experiments (performed using adifferent cell) demonstrate a maximum current density of 2 mA/cm² isattained at a potential of 2.1 V. FIG. 10 is a cyclic voltammogram (10mV/s) demonstrating the results. FIG. 11 is a graph showing some of theresults of the proof-of-concept demonstration for nitrogen generation.

In-Situ Reaction and Membrane Electrode Assembly

A Swagelok cell was fabricated using a ¾″ ID compression fitting,stainless steel current collector with gas inlet and outlet. FIG. 12 isa photograph of a portion of a disassembled, layered electrochemicalcell formed by sandwiching a Li metal sheet (250 μm thick) between anappropriate catalyst (4 mg/cm² Pt—Ru catalyst) and gasket (380 μmTeflon). When heated at an appropriate temperature (240° C. on hotplate), the nitridation reaction rapidly converted the Li metal intolithium nitride, which was obtained as a 270 μm thick purple-red disc.The lithium nitride with the catalyst and gasket represents a repeatingunit in a multilayer electrochemical stack.

Nitrogen Compression Experiment

The Swagelok cell was set up as a symmetric cell (N₂|Li₃N|N₂), operatedat a temperature of 125-150° C. with an ambient pressure of 0.77 psibeing applied on the downstream side. When an electric field wasapplied, the pressure of nitrogen gas increased in the downstream gaschamber, indicating that N₂ was being reduced, transported across thelithium nitride membrane and re-oxidized back to N₂. These results areshown in FIG. 13. When the electric field was reversed, the pressuredecreased as nitrogen was removed from the chamber of interest. Theseresults are shown in FIG. 14. This satisfactorily indicates that thesystem can be used to transport and compress nitrogen gas.

Effect of Thermal and Chemical Profiles on Membrane Formation

During membrane formation, the thermal and chemical (gas) profiles canbe adjusted/optimized to ensure proper formation. Changing thetemperature and/or gas supplied to the stack during the formationprocess may be beneficial.

For example, the temperature profile of FIG. 15 includes a short ramp toa temperature modestly higher (<10° C.) than the melting point of themetal layer. This ensures that partial melting or deformation of themetal surface and ensure improved contact between the catalyst andmembrane without deforming the bulk membrane structure. The membrane isthen rapidly cooled to a temperature just below the metal melting point.Simultaneously, the reactant gas pressure is increased to ensurecomplete conversion of the membrane. Optionally, an inert gas, such asargon, may be used when the membrane is not actively undergoingnitridation as depicted in FIG. 16.

Microstructure of Lithium Nitride Membranes Produced In Situ

Particular examples of in situ nitrided lithium nitride membranes areillustrated in FIGS. 17 and 18. FIG. 17 includes SEM micrographs of thelarger membrane (37× magnification) and (inset) a closer view (1000×magnification) of its cross-section. Energy dispersive X-ray (EDX)analysis (FIG. 18) of the nitrogen content in the membrane demonstratesconsistent nitridation was achieved across the membrane thickness. Thisparticular membrane was synthesized at a fixed reaction temperature of185° C. and 50 mm N₂ gauge pressure. The reaction temperature wasmaintained at slightly higher than the melting point of Li metal (180.5°C.), and the presence of N₂ led to the formation of a lithium nitridesolid film which helped ensure that the shape of the membrane was notdeformed.

Faradaic Efficiency and Ammonia Flux

The Faradaic efficiency for ammonia synthesis was determined in aH₂|Li₃N|N₂ electrochemical cell operated at 87° C. The potential wasincreased in steps of +0.25 V, and the current density recorded for atleast 10 minutes per step. Upon completion of the experiments, H₂ flowwas replaced with N₂, and the same voltage stepping experiment repeatedin a symmetric N₂|Li₃N|N₂ electrochemical cell. The baseline current inthe “symmetric cell” was subtracted from the current when operated usingH₂, and used to estimate the Faradaic efficiency for ammonia production.The results are shown in FIG. 19. The Faradaic efficiency was nearly100% when operated at about 0.5 V, and levels off at a value of 83% at2V, demonstrating ammonia synthesis.

It will be appreciated that variants of the above-disclosed and otherfeatures and functions, or alternatives thereof, may be combined intomany other different systems or applications. Various presentlyunforeseen or unanticipated alternatives, modifications, variations orimprovements therein may be subsequently made by those skilled in theart which are also intended to be encompassed by the following claims.

What is claimed is:
 1. A method for transporting nitride ions (N³⁻)across a solid electrolyte comprising a metal nitride in anelectrochemical cell, the electrochemical cell comprising: a firstcatalyst electrode layer; a second catalyst electrode layer; and thesolid electrolyte between the first catalyst electrode layer and thesecond catalyst electrode layer; the method comprising: applying avoltage to the electrochemical cell; providing nitrogen (N₂) to thefirst catalyst electrode layer, wherein the nitrogen reacts to formnitride ions at a surface of the first catalyst electrode layer; andtransporting the nitride ions across the solid electrolyte; wherein theelectrochemical cell is operated at a temperature in the range of from150° C. to 250° C.; wherein the nitrogen (N₂) is provided at a pressurein the range of from 1 bar to 10 bar; and wherein the solid electrolytecomprises lithium nitride.
 2. The method of claim 1, wherein at leastone of the first catalyst electrode layer and the second catalystelectrode layer comprises an alkali promoted ruthenium catalyst.
 3. Themethod of claim 1, wherein the metal nitride electrolyte was formed insitu in an electrochemical cell.
 4. The method of claim 1, wherein theelectrochemical cell is located in an electrochemical stack comprising aplurality of cells.
 5. The method of claim 1, wherein one of the firstcatalyst electrode layer and the second catalyst electrode layercomprises a ruthenium (Ru) catalyst.
 6. The method of claim 5, whereinone of the first catalyst electrode layer and the second catalystelectrode layer does not comprise the ruthenium (Ru) catalyst.